Variable electrical resistor



Oct. 11, 1960 J. G. BURNS EAL 2,956,254

VARIABLE ELECTRICAL RESISTOR Filed April 16, 1959 Fig. I 28 I lo 2 24 363 t 44 4O 2 ///J////// /AP 2 INVENTORS 2 JEROME G. BURNS 26 HARRY B.CASEY 9 f/gaz/ ATTORNEY VARIABLE ELECTRICAL RESISTOR Jerome G. Burns,Upper Darby, and Harry B. Casey,

Roslyn, Pa., assignors to International Resistance Company,Philadelphia, Pa.

Filed Apr. 16, 1959, Ser. No. 806,822

13 Claims. ((11.338-162) The present invention relates to a variableelectrical resistor, and more particularly to a variable electricalresistor having a disc contact.

In general, a variable electrical resistor, such as a rheostat orpotentiometer, includes a resistance element, and a contact whichengages the resistance element and slides therealong to vary theresistance of the variable resistor. The resistance element of such avariable resistor usually comprises a strip of an insulating materialhaving either a resistance wire helically wound therealong, or a film ofa resistance material, such as carbon or a metal, coated thereon. In avariable resistor which uses a film type resistance element, a majorproblem arises from the fact that the sliding movement of the contactover the film of resistance material wears away the film, and therebyvaries the resistance value of the resistance element. In an attempt toovercome this problem, contacts for variable resistors have beendeveloped which have rolling contact with the resistance element ratherthan a sliding contact. However, it has been found that such rollingcontacts are unsatisfactory because they do not wipe off minuteparticles of dust, dirt, or oxides which may accumulate on the surfaceof the resistance element. Such minute particles of dust, dirt, oroxides interfere with the electrical engagement between the contact andthe resistance film, and provide a noisy contact.

It is an object of the present invention to provide a novel variableelectrical resistor.

R is another object of the present invention to provide a variableelectrical resistor having a novel disc contact.

It is a further object of the present invention to provide a variableelectrical resistor having a rolling disc contact which is provided witha small but controlled amount of slippage for cleaning the resistancepath.

Other objects will appear hereinafter.

For the purpose of illustrating the invention there is shown in thedrawings a form which is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

Figure 1 is a sectional view, partly in elevation, of the variableelectrical resistor of the present invention.

Figure 2 is a sectional view taken along line 22 of Figure 1.

Referring initially to Figure 1, the variable electrical resistor of thepresent invention is generally designated as 10.

Variable resistor comprises a cup-shaped housing 12 which may be ofmetal or'an electrical insulating material. Housing 12 has a flat,circular base 14, and a cylindrical outer wall 16. A tubular bushing 18is secured to the center of the outer surface of the base 14 of housing12. As shown, bushing 18 is integral with the base 14. However, bushing18 may be separate from the housing 12, and secured to the base 14 ofthe housing 12. The outer surface of the bushing 18 is threaded toreceive a nut for mounting the variable resistor 10 on a panel.

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An annular resistance element 20 is within the housing 12, and issecured to the inner surface of the base 14 of the housing 12.Resistance element 20 comprises a fiat, annular disc 22 of an electricalinsulating material such as a ceramic or plastic, and a layer 24 of aresistance material, such as carbon or metal, coated on the outer faceof the disc 22. As shown in Figure 2, the resistance-element 28 is splitso that the resistance layer 24 provides an arcuate resistance pathhaving spaced ends. A separate terminal 26 is secured to each end of theresistance element 20, and is electrically connected to an end of theresistance layer 24. Terminals 26 project through the outer wall 16 ofthe housing 12.

A shaft 28 extends through the bushing 18 and the base 14 of the housing12, and is rotatably supported in the bushing 18. Shaft 28 has anenlarged head 30 on its end within the housing 12. The head 30 of shaft28 is disposed centrally in the space within the inner periphery of theresistance element 20. The end surface 32 of the head 30 is flat, and isinclined at an acute angle with respect to the longitudinal axis of theshaft 28. Head 36 has a recess 34- in its end surface 32. Thelongitudinal axis of the recess 34 is perpendicular to the end surface32, and is off-set from the center of the end surface 32. A stub shaft36 of an electrical insulating material, such as a plastic is rotatablysupported in the recess 34 by a ballbearing 38. A thin, circular contactdisc 40 of an electrically conductive metal is secured at its center tothe outer end of the stub shaft 36. Contact disc 40 is substantiallyparallel to the end surface 32 of the head 30 so that the contact disc40 is inclined with respect to the resistance element 20. Contact disc40 is of a diameter so that a peripheral portion of the contact disc 40engages the resistance layer 24 of the resistance element 20. As shownin Figure 2, contact disc 40 has a notch 42 extending radially from theperiphery of the contact disc 40. Notch 42 is positioned at the split inthe resistance element 20.

A helical coil spring 44 of an electrically conductive metal surroundsthe head 30 of the shaft 23, and is spaced from the head 30. One end ofthe spring 44 is secured to the contact disc 40, and the other end ofthe spring 44 is secured to a terminal 46 which extends through the base14 of housing 12. A grommet 48 of an electrical insulating material fitsbetween the terminal 46 and the base 14 of housing 12 to electricallyinsulate the term nal 46 from the housing 12. A pin 50 is secured to thebase 14- of the housing 12, and projects into the housing 12. Pin 50extends through a hole 52in the contact disc 40. A cover 54 extendsacross the housing 12, and is secured to the outer wall 16 of thehousing 12 by screws 56.

The operation of the variable resistor 10 of the present invention is asfollows:

Upon rotation of the shaft 28, the stub shaft 36 rotates with respect tothe shaft 28. Since the stub shaft 36 and the contact disc 40 areinclined with respect to the longitudinal axis of the shaft 28, rotationof the shaft 28 imparts a wobble motion to the contact disc 40. Thewobble motion of the contact disc 40 moves the point of engagementbetween the contact disc 40 and the resistance layer 24 around theperiphery of the contact disc 40, and along the length of the resistancelayer 24 to vary the electrical resistance between the terminal 46 andthe terminals 26. In essence, the wobble motion of the contact disc 40rolls the periphery of the contact disc 40 along the length of theresistance layer 24. The rolling of the periphery of the contact disc 40along the resistance layer 24 minimizes any wear of the resistance layer24 by the engagement of the contact disc 40 with the resistance layer24. The notch 42 in the contact disc 40 provides a sharp engagementbetween the contact disc 40 and the ends of the resistance layer 24, andprevents the ends of the resistance layer from being shorted out acrossthe slit in the resistance element 20 by the contact disc 40.

Since the contact disc 40 is inclined with respect to the resistanceelement 20, the path of contact on the resistance layer 24 is of asmaller diameter than the diameter of the contact disc 40. Assuming thefictional forces in the bearing 38 to be negligible, and that the pin 50does not exist, the following situation would exist. Upon rotation ofthe shaft 28 through 360 degrees, the contact disc 40 would roll alongthe resistance layer 24 in a contacting path of a diameter equal to theprojected diameter of the contacting disc 40. Because of the differencein diameter of the contacting path and the diameter of the contact disc40, a given point on the contact disc 40 would not return to itsoriginal point of engagement on the resistance layer 24, but would bedisplaced from its original point of engagement with the resistancelayer 24 a distance equal to the difference between the circumferentiallength of the contact disc 40 and the circumferential length of thecontacting path along the resistance layer 24. Therefore, in this case,the contact disc 40 would move relative to the resistance element 20upon rotation of the shaft 28. However, in the variable resistor of thepresent invention, the pin 59 prevents the contact disc 40 from rotatingwith respect to the resistance element 20. Thus, upon rotating the shaft28 through 360 degrees, a given point on the contact disc 4% will returnto its original point of engagement with the resistance layer 24. Inorder for the fixed point on the contact disc 40 to return to itsoriginal point of contact with the resistance layer 24, the contact disc40 must slide along the resistance layer 24 a distance equal to thedifference between the circumferential length of the contact disc 40 andthe circumferential length of the contacting path on the resistancelayer 24. In addition, since the intersection of the longitudinal axisof the stub shaft 36 and the plane of the contact disc 40 is off-setfrom the longitudinal axis of the shaft 28, an additional amount ofsliding is introduced between the contact disc 40 and the resistancelayer 24 upon rotation of the shaft 23. This additional amount ofsliding is proportional to the perpendicular distance between the pointof intersection between the longitudinal axis of the stub shaft 36 andthe plane of the contact disc 4t? and the longitudinal axis of the shaft23. Thus, although the contact disc 40 is rolled along the resistancelayer 24 upon rotation of the shaft 28, the variable resistor 10 of thepresent invention also provides a small amount of sliding between thecontact disc and the resistant layer 24. The small amount of slidingbetween the contact disc 40 and the resistance layer 24 is sufiicient toremove any particles of dust, dirt, or oxides which may accumulate onthe resistance layer 24, and thereby prevent a noisy contact. The amountof sliding between the contact disc 40 and the resistance layer 24 canbe controlled by the angle of inclination between the contact disc 40and the resistance element 20, and by the position of the stub shaft 36with respect to the longitudinal axis of the shaft 28 so as to providean optimum Sliding action which will cleanse the surface of theresistance layer 24, but will not cause undue wear of the resistancelayer 24. Since the various materials which can be used for theresistance layer 24 are of different hardnesses, the maximum amount ofsliding between the contact disc 40 and the resistance layer 24 whichwill be permissible will vary with the particular materials being usedfor the resistance layer 24, and can be determined experimentally.

Another advantage of the variable resistor 10 of the present inventionis that it does not require a sliding contact between the terminal 46and the contact disc 40, as is necessary in a variable resistor whichuses a rotating contact. Such a sliding contact is subject to being wornout which would break the electrical connection between the terminal andthe contact member, and also provides the variable resistor withadditional noise. However, in the variable resistor 10 of the presentinvention since the contact disc 40 does not rotate with respect to thehousing 12, the spring 44 does not wind up as a clock spring uponrotation of the shaft 28, but expands and compresses under the wobbleaction of the contact disc 40. Thus, the rotation of the shaft 28 is notlimited by the spring 44, so that the shaft 28 can be rotated as manytimes as desired in either direction. Although the variable resistor 10of the present invention is shown with the resistance element 20 beingmounted on the inner surface of the base 14 of the housing 12, theresistance element 20 can also be mounted on the inner surface of thecover 54 with the resistance layer 24 facing the contact disc 40.

The present invention may be embodied in other specific forms Withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification as indicating the scope of theinvention.

We claim:

1. In a variable resistor, an annular resistance element having aresistance material path on one planar surface thereof, a shaftrotatable about an axis which extends through the center of saidresistance element and which is perpendicular to the resistance materialpath, a flat contact disc of an electrically conducted metal rotatablymounted at its center on said shaft, said contact disc being inclined atan angle with respect to said resistance material path and being incontact with said resistance material path, and means preventingrotation of said contact disc with respect to said resistance element.

2. A variable resistor in accordance with claim 1 in which the center ofthe contact disc is off-set from the longitudinal axis of the shaft.

3. A variable resistor in accordance with claim 1 in which the shaft hasan enlarged head on its end, the end surface of said head being inclinedat an acute angle with respect to the longitudinal axis of said shaft,and the contact disc is rotatably mounted on said head and extendssubstantially parallel to the end surface of said head.

4. A variable resistor in accordance with claim 3 in which the enlargedhead on the shaft has a recess in the end surface of the head, thelongitudinal axis of said recess being perpendicular to the end surfaceof said head, a stub shaft rotatably mounted in said recess, and thecontact disc being mounted on said stub shaft.

5. A variable resistor in accordance with claim 4 in which thelongitudinal axis of the recess in the head is off-set from the centerof the end surface of the head.

6. A variable resistor comprising a housing having a cylindrical walland a substantially flat, circular wall extending across each end of thecylindrical wall, an annular resistance element mounted on the innersurface of one of the circular walls of said housing, said resistanceelement having a resistance material path on its surface facing theother of said circular Walls, a rotatable shaft extending through one ofthe circular walls of said housing, the longitudinal axis of said shaftextending through the center of said resistance element and beingsubstantially perpendicular to the resistance material path, a flatcontact disc of an electrically conducted metal within said housing androtatably mounted at its center on the end of said shaft, said contactdisc being inclined at an angle with respect to said resistance materialpath and being in contact with said resistance material path, and meansfor preventing rotation of said contact disc with respect to saidhousing.

7. A variable resistor in accordance with claim 6 in which the center ofthe contact disc is off-set from the longitudinal axis of the shaft.

8. A variable resistor in accordance with claim 6 in which the shaft hasan enlarged head on its end within the housing, the end surface of saidhead being inclined at an acute angle with respect to the longitudinalaxis of said shaft, and the contact disc is rotatably mounted on saidhead and extends substantially parallel to the end surface of said head.

9. A variable resistor in accordance with claim 8 in which the enlargedhead on the shaft has a recess in the end surface of the head, thelongitudinal axis of said recess being perpendicular to the end surfaceof said head, a stub shaft rotatably mounted in said recess, and thecontact disc being mounted on said stub shaft.

10. A variable resistor in accordance with claim 9 in which thelongitudinal axis of the recess in the head is oifset from the center ofthe end surface of the head.

11. A variable resistor in accordance with claim 10 including a terminalextending through a circular Wall of the housing, and a helical coilspring in said housing between the contact disc and said circular wallof the housing, one end of said spring being connected to said contactdisc and the other end of said spring being connected to said terminal.

12. A variable resistor in accordance With claim 11 in which theresistance element has a slit therethrough to provide a discontinuousresistance material path, and the contact disc has a notch thereinextending radially from the periphery of the contact disc, said notchbeing positioned along the slit in said resistance element.

13. A variable resistor in accordance with claim 12 in which the meanspreventing rotation of the contact disc relative to the housingcomprises a pin extending from a circular wall of the housing through ahole in said contact disc.

References Cited in the file of this patent UNITED STATES PATENTS1,704,154 Stoekle Mar. 5, 1929

